For decades, the United States has been largely responsible for Japan's defense. While it once served a purpose, this arrangement is now harmful to US interests, as well as Japan's sovereignty and its role in international relations.
Japan is guaranteed security under the US nuclear umbrella and so has not needed to take full responsibility for its own defense. While this may allay the concerns of the populace in the face of threats from other countries, it is detrimental in the long term. Without a full-fledged military, Japan lacks the diplomatic leverage necessary to become an independent player on the world stage.
Japan should also consider its sovereignty. Japanese territory is essentially at the mercy of the United States. Apart from the numerous existing US military bases considered unwelcome by many Japanese citizens, there is the possibility many more troops could arrive in the interest of US national security. This would place extreme stress on Japanese society.
Finally, there is the disadvantage to the United States. As it is obligated to defend Japan, the United States could be forced to deploy troops for conflicts it has nothing to do with, such as territorial disputes between Japan and its neighbors. This could cause bitterness between the US and Japan, and compromise the US military's ability to handle its own conflicts.
The current US-Japan military agreement is mutually disadvantageous. The United States should focus on its own military concerns, and Japan should become solely responsible for its own defense and protect its sovereignty.